The disclosure relates generally to steam turbine technology and, more particularly, to channeling leak off steam for improving steam turbine performance A related method is also provided.
A steam seal system prevents the escape of steam from and/or the entry of air into the steam turbine through the clearance between turbine shaft and casing. The steam turbine casings are equipped with packing to control the flow of leakage along the shaft. The packing is usually of a labyrinth type including of series of teeth, which are arranged to fit close to the rotating shaft with minimal clearance. The small clearance and the teeth configuration provide resistance to flow that minimizes the leakage flow along the shaft.
There are two types of packing, pressure type and vacuum type. Pressure packing seals against a positive internal steam pressure at full load and prevents the escape of steam. At part load, when a vacuum exists throughout the turbine, the packing seals against air entry into the turbine. Pressure packing arrangements have a leak-off to a steam seal header (SSH), whose pressure may be regulated by steam seal regulating valves. Pressure packing may also have one or more higher-pressure leak-offs that discharge to steam insertion points on the turbine. At full turbine load, leakage steam flows from the pressure packing into the steam seal header. At startup and part load, steam flows from the steam seal header into the packing to seal them against outside air entry into the turbines.
Vacuum packing always seal against a vacuum, regardless of turbine load. Steam must be supplied to the vacuum packing from the steam seal header. On both pressure and vacuum packing, the outermost portion of the packing is maintained at a vacuum of by a gland exhaust system (GES). A mixture of steam, from the steam seal header, and air, drawn through the outermost packing rings, is drawn to a gland condenser for heat removal and disposal to a main condenser. The gland condenser may include a shell and tube heat exchanger to condense the steam and a motor driven blower to remove the air and hold the vacuum in the system.
A steam seal feed valve (SSFV) and a steam seal dump valve (SSDV) serve to control the pressure in the steam seal header during all modes of turbine operation, from turning gear to full load. A turbine control system may monitor the pressure in the SSH and position the valves to maintain a header pressure in the desired pressure range. At startup, the entire steam turbine is at vacuum. All the turbine packings require steam to be fed into them from the steam seal header. Steam from an external source (usually an auxiliary boiler) is fed to the SSH, under the control of the SSFV. The SSDV is closed during this operation.
The flow to the pressure packing normally decreases proportional to turbine load, until eventually the flow direction reverses and it starts feeding steam into the SSH. The flow to the vacuum packings is approximately constant irrespective of load. The load point, at which the flow from the pressure packings equals the flow into the vacuum packings, is called the “self-sealing” point. As load increases above the self-sealing point, the SSDV sets itself open to control the SSH pressure by dumping the excess steam to the main condenser with the SSFV now closed. The external steam source is isolated in this condition by closing the SSFV.
A further leak off path may be provided on the high pressure side of the shaft packing for the High Pressure (HP) turbine inlet side, the HP turbine outlet side and the Intermediate Pressure (IP) turbine inlet side. The associated leak off lines may be operatively connected for delivery of the leak off steam to various locations in the steam turbine system for use in power production. This leak off steam may be provided to such locations as a vertical joint between the IP/LP turbines, LP turbine steam admission pipe and shell stages of one of the HP turbine, IP turbine and the LP turbine.
Current steam seal systems are of a single set point sub-optimized design. For example, these designs for the turbine arrangement described above may provide a self-sealing load point (“SSLP”) of about (30-45%). When a steam turbine “self seals”, the terms generally refer to the condition where pressure packing seal steam flow is sufficient to pressurize and seal the vacuum packings. In higher load conditions however, the pressure packing steam flow going to the steam seal header increases but the vacuum packing requirement may be approximately constant. The additional steam coming from the pressure packings into the steam seal system thus may be dumped to the condenser using a SSDV without extracting any work.
Steam turbines are a relatively mature technology where efficiency improvements have great importance in a competitive market. Performance improvements with minimum additional cost are desirable from a competitive standpoint.